In multi-wavelength photoexcitation spectroscopy, independent control of multiple monochromatic excitation wavelengths is often required to characterize the wavelength-dependent optical properties in various media, such as semiconductors, biological chemicals, volatile organic vapors, ceramics, thin films, biological tissues, and the like. To classify as “monochromatic,” the optical excitation wavelength must be spectrally narrow with respect to the spectral properties of the sample, providing high spectral resolution. Examples of such wavelength-dependent (“spectral”) properties include optical absorption, fluorescence, transmission, reflection, polarization, and the like. Other high resolution spectral properties include the conversion of optical energy to other energies, such as with Raman spectroscopy, photoacoustic spectroscopy, photomagnetic spectroscopy, and the like. Because of the inhomogenities and/or size constraints that exist in many samples, a tight optical focus, often less than 100 μm in diameter, may be required to accurately assess the optical properties from background noise. In such case, all optical excitation wavelengths must focus at the same narrow point on a sample to provide accurate optical characterization.
To implement multi-wavelength photoexcitation spectroscopy, a variety of optical source-detector configurations can be employed. For example, a broadband optical source, such as a tungsten-halogen, mercury-arc, or xenon lamp, can be integrated into an optical spectrometer. In this case, the spectral width of the excitation beam can be controlled by manipulating the optical slit width and by implementing a high- or low-resolution optical diffraction grating. Unfortunately, this technique may be time consuming, as characterizing the optical properties of a given sample may be limited by the rotational speed of the optical diffraction grating. Furthermore, characterizing time-dependent optical properties may be compromised by the slow switching speed of the broadband sources. Lastly, being bulky, power-hungry, and delicate, this setup may be unsuitable for spectroscopy applications requiring user-portable, and perhaps wearable, spectrometry tools. Replacing the lamps with multiple lasers may eliminate problems associated with speed, as a diffraction grating is not needed, but adding lasers may compound problems associated with system cost and portability.
To increase portability and reduce cost, a single optical source, such as a laser diode (LD) can be configured as a controllable multi-wavelength emitter through a high-speed, electrically controllable Bragg reflector, via the electrooptic effect. By adjusting the voltage of the electrooptic electrodes, the peak excitation wavelength can be selected for spectroscopy applications. However, the spectral range of the peak wavelength, being limited by the maximum index of refraction differential, may be restricted to a few nanometers at best, and this may not be suitable for many spectroscopy applications. Broadening the spectral range can be achieved by introducing multiple laser diodes (LDs), light-emitting diodes (LEDs), or other compact optical point sources, but this may add complexity to a compact system. Namely, each additional wavelength range may not only require an additional optical emitter but also may require additional packaging and optical alignment. For example, in the case of LDs or LEDs, each optical wavelength range, from UV to visible to IR, may require a different semiconductor material system, each requiring its own packaging and optics. Moreover, each LED or LD may require optical alignment with respect to the other.